It will be understood within such machinery as gas turbine engines it is necessary to monitor a number of conditions in order to maintain operational efficiency. It will be appreciated that these conditions may relate to current output from the machinery, its temperature or other operational conditions such as stress loading. In such circumstances sensors are provided within and about the machinery such as a gas turbine engine in order to provide responses to control apparatus or indicator displays or otherwise for utilisation within the overall control strategy for the machinery.
With regard to engines it will be appreciated that some conditions are operationally critical. Thus, it is important that the engine or machinery remains within operational parameters such that the machinery will not malfunction or catastrophically fail. In such circumstances it is known to provide a number of sensors to effectively repeat or provide redundancy with regard to sensor determinations or averaging. These sensors are arranged in sensor channels extending about the machinery. For example within a gas turbine engine it is known to provide a sensor ladder having a number of thermo couples connected in parallel such that the responses from the thermo couples can be combined and monitored by a measurement device. Typically, this sensor ladder as indicated has the sensors, that is to say the thermocouples, connected in parallel such that measurement signals or readings can be taken from the respective ends of the sensor ladder. In such circumstances, the measurement system can determine if there is a disparity between the sensor signals at each end of the sensor ladder and so highlight a potential error. Unfortunately, some sensor failures may not result in appropriate disparity or differences in the signal at respective ends of the channel (sensor ladder).
It will be appreciated if signals received at each end of the sensor ladder agree within a predetermined threshold amount then the average of the two is used as indication signals for subsequent control or indication purposes. However, by having all sensors connected in a single parallel ladder, it will be understood that a single sensor or connecting harness failure will affect the whole system and it is not always possible, even with dual channel measurement of the ladder to establish that there is even a fault. If a difference beyond the threshold level between the channels occurs then it is not possible to determine which of the sensor channels is correct. It will be understood that measurement from the ends of each sensor ladder is also not an accurate assessment of the average sensor signal values. The sensor signals presented to the measurement device in the ladder are typically biased towards the sensors nearer to the end of the ladder as the electrical connections in the ladder provide an electrical resistance which is not negligible compared with sensor electrical resistance.
For the above reasons previous sensor apparatus and methods of monitoring sensors although mostly acceptable can provide spurious results.